The present invention relates to security documents including a security image area composed of a printed security image and a printed complementary security image. The security image area is arranged to provide an indication of document authenticity.
For example, the security image elements and the complementary security image elements may be designed such that an attempted duplication or reproduction results in the formation of a readily apparent warning image on the face of the duplicate document. The security image elements and the complementary security image elements are arranged such that the presence of the security image and the security image elements is not readily apparent on the original. Examples of security documents of this type are illustrated in U.S. Pat. Nos. 4,579,370, 5,149,140, 5,197,765, 5,340,159, the disclosures of which are incorporated herein by reference.
Typically, conventional security documents are constructed by utilizing what is commonly referred to as an orthogonal halftone printing screen. The screen enables production of an orthogonal array of printed dots across the face of the document. Two different screens may be superimposed to produce a security image wherein a security image or warning term is superimposed on a background image. Although the traditional orthogonal printing screens have enjoyed significant commercial use, this conventional security document printing scheme is somewhat limited in its versatility and efficiency.
Further, many conventional security documents include a security image consisting of an orthogonal array of printed elements (see FIG. 5) The orthogonal array is, in many cases, convenient to produce because it complements many of the existing document design schemes available today, including orthogonal halftone screening. However, in certain circumstances, a security image constructed of printed units arranged in an orthogonal array may be more easily recognized by the naked eye or more easily reproduced by duplicating equipment.
Accordingly, there is a need for a more versatile and efficient security document production scheme, preferably one that takes advantage of the computing power available throughout the document production industry and one that does not utilize a conventional orthogonal array of printed elements.